The field of the invention is micro electromechanical (MEM) capacitors.
Various devices would benefit from capacitors with a wide tuning range. Wireless communication systems, for example, would benefit from tunable capacitors having a wide tuning range and low loss. A useful capacitor should also be capable of being monolithically integrated with other IC circuits.
Tunable capacitors are widely used in RF communication applications for low-noise paramteric amplifiers, harmonic frequency generators, and frequency controllers such as voltage-controlled oscillators. Typically, solid-state veractors used for these devices. However, solid-state veractors provide a very limited tuning range with high resistive loss.
MEM capacitors offer better range and other advantages. MEM capacitors can achieve higher quality factor compared to CMOS veractors. Integration of MEM capacitors provides lower interconnection and parasitic related losses. In addition, the MEM capacitors can reduce complexity due to monolithic integration. Conventionally, MEM capacitors have a mechanically suspended plate suspended over a fixed plate. A bias voltage is used to vary the distance between the two parallel plates, and accordingly vary the capacitance. However, the adjustable range of the conventional MEM tunable capacitor is limited by the pull-in effect. This effect limits how close the parallel plates may be brought together. The conventional devices are limited to a tunable range defined by one third of the distance between the parallel plates. Once spacing is reduced by one third, the pull in effect causes the plates to be brought together. An explanation of this limit is included in the description of a MEM capacitor developed by Young and Boser in xe2x80x9cA Micromachined Variable Capacitor for Monolithic Low-Noise VCOS,xe2x80x9d Tech. Digest of Solid State Sensors and Actuator Workshop, Hilton Head, S.C., Jun. 2-6, 1996, pp. 124-127.
A MEM capacitor having a structure that permitted a wider tuning range would be beneficial. It is an object of the present invention to provide such a capacitor.
The present invention provides a capacitor structure having a capacitance plate nearer a movable plate than a separate bias plate. Voltage potential between the bias plate and movable plate determines the value of capacitance between the movable plate and the capacitance plate.
In a preferred embodiment MEMs capacitor, the movable plate is suspended over two fixed plates, a bias plate and a capacitance plate. The movable plate is disposed opposite both the bias plate and the capacitance plate. A distance between opposing surfaces of the capacitance plate and the movable plate is less than a distance between the bias plate and the capacitance plate. Preferably, the relative difference in distances between the plates is accomplished by a mechanically suspended movable plate that is shaped to have portions in at least two separate planes.